Knee (construction)

In woodworking, a knee is a natural or cut curved piece of wood. Knees, sometimes called ship's knees, are a common form of bracing in boat building and occasionally in timber framing. A knee rafter in carpentry is a bent rafter used to gain head room in an attic.

Strength characteristics

Wood is a highly anisotropic material (its strength varies considerably with the direction of applied force, i.e., parallel, radial, or tangential to the grain). Because wood is strongest when loaded in tension or compression along the grain, the best knees are those in which the wood grain follows the bend. For a knee with relatively little bend, it may be possible to cut the knee out of a single straight-grained board and still achieve sufficient strength. However, with increasing bend this method becomes problematic since more and more of the knee is aligned across the grain and is therefore considerably weaker. A knee laid out this way might easily snap in two under hand pressure alone, even if it is generously sized. In boat joinery constantly subject to shock and fatigue loading this method is unsuitable.

To avoid this issue knees requiring sharper curves are made using methods which ensure that the wood grain and direction of load are closely aligned.